The reaction of N-heterocyclic carbenes with polyaminoboranes [MeNH–BH2]n or [NH2–BH2]n at 20 °C led to depolymerisation and the formation of labile, monomeric aminoborane–NHC adducts, RNH–BH2–NHC (R = Me or H); a similar NHC adduct of Ph2NBCl2 was characterized by single crystal X-ray diffraction.